The present invention relates in general to processing materials, and more particularly to processing substrates for electronics, optics, or optoelectronics. More precisely, the invention relates to apparatus enabling two wafers of material initially adjoining each other to be split apart in a splitting plane. The invention also relates to a method implemented by such apparatus.
As explained below, the invention applies to wafers of material which adjoin with varying degrees of cohesion. The term “adjoining” wafers is used to mean wafers positioned in such a manner as to be in physical contact. Thus, “adjoining” wafers can still be connected together by structural elements (in particular for wafers that define between them a plane of weakness as described below, which plane of weakness has been subjected to treatment, e.g. heat treatment, that does not achieve complete cleaving between the two wafers on either side of the plane); or on the contrary need not be connected together by any structural element, but instead are held together solely by means of forces such as those corresponding to a “suction-cup” effect between the adjoining but disjoint surfaces of the two wafers, or by Van der Waals forces (in particular to wafers coming initially from the same substrate, with a plane of weakness being defined between them, which plane of weakness has been subjected to heat or other treatment enabling complete cleaving to be obtained between the two materials on either side of the plane of weakness). The two adjoining wafers thus form an assembly referred to below as a substrate.
A known method of manufacturing SOI type substrates which is known under the generic term of the SMARTCUT® method, comprises the following principal steps:                a step of implanting species (ions or atoms) to a given depth in a substrate of semiconductor material such as monocrystalline silicon, so as to define a plane of weakness in the substrate;        a step of fixing the substrate to a stiffener, such as, for example, silicon that has optionally been oxidized on its surface; and then        a cleaving step for imparting a fracture in at least a part of the plane of weakness defined at the level of the layer of implanted species. There is thus one wafer on either side of the fracture (a first wafer corresponding to the portion of the monocrystalline silicon substrate which has been detached and possibly also fixed to a stiffener so as to constitute the SOI, and a second wafer which corresponds to the remainder of the silicon).        
In some cases, if the two wafers of substrate situated on either side of the plane of weakness are sufficiently thick themselves to present some minimum level of strength, then the step of fixing to a stiffener can be omitted.
It is also possible to grow a silicon substrate directly on the stiffener instead of fixing the substrate to the stiffener.
An example of a general description of a method of that type can be found in U.S. Pat. No. 5,374,564. In that type method, regardless of whether the fracture implemented during the cleaving step is complete or only partial, the two wafers remain, in practice, adhering to each other merely by the suction-cup effect when the fracture is complete, given that the faces of the wafers are extremely smooth.
It thus remains necessary in these cases to finally split them apart in order to obtain firstly the final SOI substrate and secondly the remainder of monocrystalline silicon, which can be recycled in the method.
It is also recalled that although that patent generally discusses splitting apart pairs of wafers in which one of the wafers corresponds to SOI, there are other requirements for splitting or separating wafers that are adhered or adjoined to each other.
The problem of adhesion between two wafers, even if only by the suction-cup effect, is to be found in numerous two-wafer structures, and for this reason it is necessary to split them. Thus, any or a wide variety of different wafer structures exhibit this feature, including the known SOI, SOA, or AOA types, and splitting is required to separate the wafers for further processing or use.
When the dimensions (in particular the thickness) of the wafers make them fragile, and/or when the material of the wafers is not strong, the two wafers must naturally be split apart while taking very great precautions to avoid running the risk of damaging the two wafers as the wafers are being moved apart from each other, where necessary while also implementing or finishing off the fracture.
This applies in particular when splitting an SOI substrate apart from the remaining monocrystalline silicon. In this application, the splitting-apart operation is generally performed manually by an operator who is particularly skilled. For example, the operator can insert a sharp blade or the like into the edge of the silicon substrate level with a cleavage plane (which corresponds to the plane of weakness), thus making it possible by a wedging effect to split the wafers apart. That operation runs the risk of inducing impacts or rubbing between the facing faces of the two wafers, and thus of damaging them. In addition, that manual operation is lengthy and fiddly, and rates of production throughput depend thereon to a great extent.
Finally, particularly when the fracturing between the two wafers needs to be finished off by the process of splitting the wafers apart, the forces delivered to the substrate need to be large, and the above-mentioned manual operation becomes ill-suited or even dangerous. It will thus be understood that it would be particularly advantageous to be able to propose a method and apparatus enabling wafers to be split apart quickly, reliably, and reproducibly, and which also avoids any contact between the faces of the wafers while they are being split apart so as to prevent any risk of scratching or of particles being deposited on the faces of the wafers.
In this regard, U.S. Pat. No. 6,468,679 describes a method and apparatus seeking to satisfy that need. While this is useful for its intended purpose, there still is a need for further improvements in this area.
Other apparatuses are known for the purpose of splitting wafers apart from a semiconductor substrate. For example, U.S. Pat. No. 6,382,292 discloses an apparatus seeking to split apart a wafer by a fluid jet. It should be observed that the technology implemented for fluid jet splitting is specific and corresponds to a very particular option. One limitation associated with that option is the need to provide sophisticated means for holding the substrate to be split apart in extremely accurate manner relative to a jet of fluid which is applied to the edge of the substrate. That requires the wafer faces that are to be split apart to be subjected to stresses that can generate irregularities in the structure of the wafers after splitting. In addition, that patent document does not provide any solution that would enable a plurality of wafers that have just been split apart to be handled in simple manner.
JP 04 010 454 also relates to a technique of splitting wafers apart by a fluid jet. That document proposes a highly specific sequence seeking to split wafers apart individually from substrates. In particular, provision is made for selectively turning over certain substrates, thereby making the splitting method more complex.
U.S. Pat. No. 6,427,748 also describes a variant in which use is made of ultrasound for splitting wafers apart from a substrate. In that case also, the technology used is very specific to the intended use. Furthermore, entirely particular means need to be provided and a complex method of operation is associated with such treatment. In particular, it is necessary to place the substrate that is to be treated in an enclosure for exposing it to ultrasound, and then to remove the separated wafers from the enclosure, corresponding to handling operations that are fiddly.
Mention can also be made, for information purposes, of document JP 09 069 552 as a document that teaches no more than an apparatus for handling substrates, but not for splitting wafers apart.
In view of the above, it is clear that there is a need for a new method and apparatus for splitting adjoined substrates, and these are now provided by the present invention.